/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;


import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Set;


/**
 * A collection that supports order-independent equality, like {@link Set}, but
 * may have duplicate elements. A multiset is also sometimes called a
 * <i>bag</i>.
 *
 * <p>Elements of a multiset that are equal to one another are referred to as
 * <i>occurrences</i> of the same single element. The total number of
 * occurrences of an element in a multiset is called the <i>count</i> of that
 * element (the terms "frequency" and "multiplicity" are equivalent, but not
 * used in this API). Since the count of an element is represented as an {@code
 * int}, a multiset may never contain more than {@link Integer#MAX_VALUE}
 * occurrences of any one element.
 *
 * <p>{@code Multiset} refines the specifications of several methods from
 * {@code Collection}. It also defines an additional query operation, {@link
 * #count}, which returns the count of an element. There are five new
 * bulk-modification operations, for example {@link #add(Object, int)}, to add
 * or remove multiple occurrences of an element at once, or to set the count of
 * an element to a specific value. These modification operations are optional,
 * but implementations which support the standard collection operations {@link
 * #add(Object)} or {@link #remove(Object)} are encouraged to implement the
 * related methods as well. Finally, two collection views are provided: {@link
 * #elementSet} contains the distinct elements of the multiset "with duplicates
 * collapsed", and {@link #entrySet} is similar but contains {@link Entry
 * Multiset.Entry} instances, each providing both a distinct element and the
 * count of that element.
 *
 * <p>In addition to these required methods, implementations of {@code
 * Multiset} are expected to provide two {@code static} creation methods:
 * {@code create()}, returning an empty multiset, and {@code
 * create(Iterable<? extends E>)}, returning a multiset containing the
 * given initial elements. This is simply a refinement of {@code Collection}'s
 * constructor recommendations, reflecting the new developments of Java 5.
 *
 * <p>As with other collection types, the modification operations are optional,
 * and should throw {@link UnsupportedOperationException} when they are not
 * implemented. Most implementations should support either all add operations
 * or none of them, all removal operations or none of them, and if and only if
 * all of these are supported, the {@code setCount} methods as well.
 *
 * <p>A multiset uses {@link Object#equals} to determine whether two instances
 * should be considered "the same," <i>unless specified otherwise</i> by the
 * implementation.
 *
 * <p>Common implementations include {\@link ImmutableMultiset}, {\@link
 * HashMultiset}, and {\@link ConcurrentHashMultiset}.
 *
 * <p>If your values may be zero, negative, or outside the range of an int, you
 * may wish to use {\@link com.google.common.util.concurrent.AtomicLongMap}
 * instead. Note, however, that unlike {@code Multiset}, {@code AtomicLongMap}
 * does not automatically remove zeros.
 *
 * <p>See the Guava User Guide article on <a href=
 * "https://github.com/google/guava/wiki/NewCollectionTypesExplained#multiset">
 * {@code Multiset}</a>.
 *
 * @author Kevin Bourrillion
 * @since 2.0
 */

public interface Multiset<E> extends Collection<E> {
    // Query Operations

    /**
     * Returns the number of occurrences of an element in this multiset (the
     * <i>count</i> of the element). Note that for an {@link Object#equals}-based
     * multiset, this gives the same result as {@link Collections#frequency}
     * (which would presumably perform more poorly).
     *
     * <p><b>Note:</b> the utility method {\@link Iterables#frequency} generalizes
     * this operation; it correctly delegates to this method when dealing with a
     * multiset, but it can also accept any other iterable type.
     *
     * @param element the element to count occurrences of
     * @return the number of occurrences of the element in this multiset; possibly
     *     zero but never negative
     */
    int count(Object element);

    // Bulk Operations

    /**
     * Adds a number of occurrences of an element to this multiset. Note that if
     * {@code occurrences == 1}, this method has the identical effect to {@link
     * #add(Object)}. This method is functionally equivalent (except in the case
     * of overflow) to the call {@code addAll(Collections.nCopies(element,
     * occurrences))}, which would presumably perform much more poorly.
     *
     * @param element the element to add occurrences of; may be null only if
     *     explicitly allowed by the implementation
     * @param occurrences the number of occurrences of the element to add. May be
     *     zero, in which case no change will be made.
     * @return the count of the element before the operation; possibly zero
     * @throws IllegalArgumentException if {@code occurrences} is negative, or if
     *     this operation would result in more than {@link Integer#MAX_VALUE}
     *     occurrences of the element
     * @throws NullPointerException if {@code element} is null and this
     *     implementation does not permit null elements. Note that if {@code
     *     occurrences} is zero, the implementation may opt to return normally.
     */
    int add(E element, int occurrences);

    /**
     * Removes a number of occurrences of the specified element from this
     * multiset. If the multiset contains fewer than this number of occurrences to
     * begin with, all occurrences will be removed.  Note that if
     * {@code occurrences == 1}, this is functionally equivalent to the call
     * {@code remove(element)}.
     *
     * @param element the element to conditionally remove occurrences of
     * @param occurrences the number of occurrences of the element to remove. May
     *     be zero, in which case no change will be made.
     * @return the count of the element before the operation; possibly zero
     * @throws IllegalArgumentException if {@code occurrences} is negative
     */
    int remove(Object element, int occurrences);

    /**
     * Adds or removes the necessary occurrences of an element such that the
     * element attains the desired count.
     *
     * @param element the element to add or remove occurrences of; may be null
     *     only if explicitly allowed by the implementation
     * @param count the desired count of the element in this multiset
     * @return the count of the element before the operation; possibly zero
     * @throws IllegalArgumentException if {@code count} is negative
     * @throws NullPointerException if {@code element} is null and this
     *     implementation does not permit null elements. Note that if {@code
     *     count} is zero, the implementor may optionally return zero instead.
     */
    int setCount(E element, int count);

    /**
     * Conditionally sets the count of an element to a new value, as described in
     * {@link #setCount(Object, int)}, provided that the element has the expected
     * current count. If the current count is not {@code oldCount}, no change is
     * made.
     *
     * @param element the element to conditionally set the count of; may be null
     *     only if explicitly allowed by the implementation
     * @param oldCount the expected present count of the element in this multiset
     * @param newCount the desired count of the element in this multiset
     * @return {@code true} if the condition for modification was met. This
     *     implies that the multiset was indeed modified, unless
     *     {@code oldCount == newCount}.
     * @throws IllegalArgumentException if {@code oldCount} or {@code newCount} is
     *     negative
     * @throws NullPointerException if {@code element} is null and the
     *     implementation does not permit null elements. Note that if {@code
     *     oldCount} and {@code newCount} are both zero, the implementor may
     *     optionally return {@code true} instead.
     */
    boolean setCount(E element, int oldCount, int newCount);

    // Views

    /**
     * Returns the set of distinct elements contained in this multiset. The
     * element set is backed by the same data as the multiset, so any change to
     * either is immediately reflected in the other. The order of the elements in
     * the element set is unspecified.
     *
     * <p>If the element set supports any removal operations, these necessarily
     * cause <b>all</b> occurrences of the removed element(s) to be removed from
     * the multiset. Implementations are not expected to support the add
     * operations, although this is possible.
     *
     * <p>A common use for the element set is to find the number of distinct
     * elements in the multiset: {@code elementSet().size()}.
     *
     * @return a view of the set of distinct elements in this multiset
     */
    Set<E> elementSet();

    /**
     * Returns a view of the contents of this multiset, grouped into {@code
     * Multiset.Entry} instances, each providing an element of the multiset and
     * the count of that element. This set contains exactly one entry for each
     * distinct element in the multiset (thus it always has the same size as the
     * {@link #elementSet}). The order of the elements in the element set is
     * unspecified.
     *
     * <p>The entry set is backed by the same data as the multiset, so any change
     * to either is immediately reflected in the other. However, multiset changes
     * may or may not be reflected in any {@code Entry} instances already
     * retrieved from the entry set (this is implementation-dependent).
     * Furthermore, implementations are not required to support modifications to
     * the entry set at all, and the {@code Entry} instances themselves don't
     * even have methods for modification. See the specific implementation class
     * for more details on how its entry set handles modifications.
     *
     * @return a set of entries representing the data of this multiset
     */
    Set<Entry<E>> entrySet();

    /**
     * An unmodifiable element-count pair for a multiset. The {@link
     * Multiset#entrySet} method returns a view of the multiset whose elements
     * are of this class. A multiset implementation may return Entry instances
     * that are either live "read-through" views to the Multiset, or immutable
     * snapshots. Note that this type is unrelated to the similarly-named type
     * {@code Map.Entry}.
     *
     * @since 2.0
     */
    interface Entry<E> {

        /**
         * Returns the multiset element corresponding to this entry. Multiple calls
         * to this method always return the same instance.
         *
         * @return the element corresponding to this entry
         */
        E getElement();

        /**
         * Returns the count of the associated element in the underlying multiset.
         * This count may either be an unchanging snapshot of the count at the time
         * the entry was retrieved, or a live view of the current count of the
         * element in the multiset, depending on the implementation. Note that in
         * the former case, this method can never return zero, while in the latter,
         * it will return zero if all occurrences of the element were since removed
         * from the multiset.
         *
         * @return the count of the element; never negative
         */
        int getCount();

        /**
         * {@inheritDoc}
         *
         * <p>Returns {@code true} if the given object is also a multiset entry and
         * the two entries represent the same element and count. That is, two
         * entries {@code a} and {@code b} are equal if: <pre>   {@code
         *
         *   Objects.equal(a.getElement(), b.getElement())
         *       && a.getCount() == b.getCount()}</pre>
         */
        @Override
        // TODO(kevinb): check this wrt TreeMultiset?
        boolean equals(Object o);

        /**
         * {@inheritDoc}
         *
         * <p>The hash code of a multiset entry for element {@code element} and
         * count {@code count} is defined as: <pre>   {@code
         *
         *   ((element == null) ? 0 : element.hashCode()) ^ count}</pre>
         */
        @Override
        int hashCode();

        /**
         * Returns the canonical string representation of this entry, defined as
         * follows. If the count for this entry is one, this is simply the string
         * representation of the corresponding element. Otherwise, it is the string
         * representation of the element, followed by the three characters {@code
         * " x "} (space, letter x, space), followed by the count.
         */
        @Override
        String toString();
    }

    // Comparison and hashing

    /**
     * Compares the specified object with this multiset for equality. Returns
     * {@code true} if the given object is also a multiset and contains equal
     * elements with equal counts, regardless of order.
     */
    @Override
    // TODO(kevinb): caveats about equivalence-relation?
    boolean equals(Object object);

    /**
     * Returns the hash code for this multiset. This is defined as the sum of
     * <pre>   {@code
     *
     *   ((element == null) ? 0 : element.hashCode()) ^ count(element)}</pre>
     *
     * <p>over all distinct elements in the multiset. It follows that a multiset and
     * its entry set always have the same hash code.
     */
    @Override
    int hashCode();

    /**
     * {@inheritDoc}
     *
     * <p>It is recommended, though not mandatory, that this method return the
     * result of invoking {@link #toString} on the {@link #entrySet}, yielding a
     * result such as {@code [a x 3, c, d x 2, e]}.
     */
    @Override
    String toString();

    // Refined Collection Methods

    /**
     * {@inheritDoc}
     *
     * <p>Elements that occur multiple times in the multiset will appear
     * multiple times in this iterator, though not necessarily sequentially.
     */
    @Override
    Iterator<E> iterator();

    /**
     * Determines whether this multiset contains the specified element.
     *
     * <p>This method refines {@link Collection#contains} to further specify that
     * it <b>may not</b> throw an exception in response to {@code element} being
     * null or of the wrong type.
     *
     * @param element the element to check for
     * @return {@code true} if this multiset contains at least one occurrence of
     *     the element
     */
    @Override
    boolean contains(Object element);

    /**
     * Returns {@code true} if this multiset contains at least one occurrence of
     * each element in the specified collection.
     *
     * <p>This method refines {@link Collection#containsAll} to further specify
     * that it <b>may not</b> throw an exception in response to any of {@code
     * elements} being null or of the wrong type.
     *
     * <p><b>Note:</b> this method does not take into account the occurrence
     * count of an element in the two collections; it may still return {@code
     * true} even if {@code elements} contains several occurrences of an element
     * and this multiset contains only one. This is no different than any other
     * collection type like {@link List}, but it may be unexpected to the user of
     * a multiset.
     *
     * @param elements the collection of elements to be checked for containment in
     *     this multiset
     * @return {@code true} if this multiset contains at least one occurrence of
     *     each element contained in {@code elements}
     * @throws NullPointerException if {@code elements} is null
     */
    @Override
    boolean containsAll(Collection<?> elements);

    /**
     * Adds a single occurrence of the specified element to this multiset.
     *
     * <p>This method refines {@link Collection#add}, which only <i>ensures</i>
     * the presence of the element, to further specify that a successful call must
     * always increment the count of the element, and the overall size of the
     * collection, by one.
     *
     * <p>To both add the element and obtain the previous count of that element,
     * use {@link #add(E, int) add}{@code (element, 1)} instead.
     *
     * @param element the element to add one occurrence of; may be null only if
     *     explicitly allowed by the implementation
     * @return {@code true} always, since this call is required to modify the
     *     multiset, unlike other {@link Collection} types
     * @throws NullPointerException if {@code element} is null and this
     *     implementation does not permit null elements
     * @throws IllegalArgumentException if {@link Integer#MAX_VALUE} occurrences
     *     of {@code element} are already contained in this multiset
     */
    @Override
    boolean add(E element);

    /**
     * Removes a <i>single</i> occurrence of the specified element from this
     * multiset, if present.
     *
     * <p>This method refines {@link Collection#remove} to further specify that it
     * <b>may not</b> throw an exception in response to {@code element} being null
     * or of the wrong type.
     *
     * <p>To both remove the element and obtain the previous count of that element,
     * use {@link #remove(E, int) remove}{@code (element, 1)} instead.
     *
     * @param element the element to remove one occurrence of
     * @return {@code true} if an occurrence was found and removed
     */
    @Override
    boolean remove(Object element);

    /**
     * {@inheritDoc}
     *
     * <p><b>Note:</b> This method ignores how often any element might appear in
     * {@code c}, and only cares whether or not an element appears at all.
     * If you wish to remove one occurrence in this multiset for every occurrence
     * in {@code c}, see {\@link Multisets#removeOccurrences(Multiset, Multiset)}.
     *
     * <p>This method refines {@link Collection#removeAll} to further specify that
     * it <b>may not</b> throw an exception in response to any of {@code elements}
     * being null or of the wrong type.
     */
    @Override
    boolean removeAll(Collection<?> c);

    /**
     * {@inheritDoc}
     *
     * <p><b>Note:</b> This method ignores how often any element might appear in
     * {@code c}, and only cares whether or not an element appears at all.
     * If you wish to remove one occurrence in this multiset for every occurrence
     * in {@code c}, see {\@link Multisets#retainOccurrences(Multiset, Multiset)}.
     *
     * <p>This method refines {@link Collection#retainAll} to further specify that
     * it <b>may not</b> throw an exception in response to any of {@code elements}
     * being null or of the wrong type.
     *
     * \@see Multisets#retainOccurrences(Multiset, Multiset)
     */
    @Override
    boolean retainAll(Collection<?> c);
}
